Adaptive frame durations for time-hopped impulse radio systems

ABSTRACT

A method determines a duration of a frame in a time-hopping, impulse radio system. Channel state information of a channel between two transceivers is estimated periodically. A signal to noise and interference ratio is also estimated periodically. The frame duration of the frame is then determined according to the channel state information and the signal to noise and interference ratio.

FIELD OF THE INVENTION

The invention relates generally to communication systems, and moreparticularly to modulation formats used in wireless communicationsystems.

BACKGROUND OF THE INVENTION

In the United States, the Federal Communications Commission (FCC) allowsa restricted unlicensed use of ultra-wide bandwidth (UWB) signals forwireless communication systems, “First Report and Order,” Feb. 14, 2002.The UWB signals must be in the frequency range from 3.1 to 10.6 GHz, andhave a minimum bandwidth of 500 MHz. The FCC order also limits the powerspectral density and peak emissions power of UWB signals to less than−43.1 dBm/MHz.

One modulation method for UWB uses extremely short time pulses, e.g.,1/1,000,000,000 of a second or less, to generate signals with bandwidthsgreater than 500 MHz which corresponds to a wavelength of about 300 mm.Wireless systems that use short pulses are commonly referred to asimpulse radio (IR) systems.

As shown in FIG. 1A, four different modulation techniques are commonlyused for IR systems: pulse position modulation (PPM) 11, pulse amplitudemodulation (PAM) 12, on-off keying (OOK) 13, and bi-phase shift keying(BPSK) 14.

As an advantage, UWB systems achieve high data rates, and are resistantto multi-path impairments. This is due to large processing gains.Additionally, IR systems enable low cost, low duty cycle, low powertransceivers that do not require local oscillators for heterodyning.Because UWB transceivers are primarily implemented in the digitaldomain, the UWB transceivers can be integrated in a semiconductor chip.In UWB systems, multiple transceivers concurrently share the samespectrum without interference. UWB systems are ideal for short range,high-speed networks in homes, businesses, and educational institutions.Sensor networks can also use UWB transceivers.

A time-hopping (TH) IR is described by M. Win and R. A. Scholtz,“Ultra-Wide Band Width Time-Hopping Spread-Spectrum Impulse Radio forWireless Multiple-Access Communications,” in IEEE Trans. OnCommunications, Vol. 48, No. 4 April 2000, pp. 679-691. In that TH-IRsystem, each bit or symbol is represented by N_(f) pulses, where N_(f)is a positive integer. The time to transmit a bit is T_(s). This iscalled the symbol duration. The time T_(s) is further partitioned intoframes T_(f), and the frames are partitioned into chips T_(c),corresponding typically to a pulse duration. If N_(c) represents thenumber of chips in a frame and N_(f) represents the number of frames ina symbol, then T_(s), T_(f), and T_(c) are related byT_(s)=N_(f)T_(f)=N_(f)N_(c)T_(c).   (1)

FIG. 1B shows the relationship between the symbol time T_(s) 101, theframe duration T_(f) 102, and the chip duration T_(c) 103 for pulses 104for an example prior art TH-IR waveform 110 for a ‘0’ bit, and awaveform 120 for a ‘1’ bit. Typically, the pulses are spacedpseudo-randomly among the available chips in a frame according to a“time-hopping” code to minimize multi-user interference.

As stated above, the modulation can be binary phase shift keying (BPSK).With BPSK, each bit b is represented as either a positive or negativeone, i.e., b ∈{−1, 1}. The transmitted signal has the form$\begin{matrix}{{{s(t)} = {\sum\limits_{i = 1}^{\infty}{\sum\limits_{j = 1}^{N_{f}}{h_{i,j}b_{\lfloor{i/N_{f}}\rfloor}{p\left( {t - {j\quad T_{f}} - {c_{j}T_{c}}} \right)}}}}},} & (2)\end{matrix}$where c_(j) represents the j^(th) value of the TH code, in a range {0,1, . . . , N_(c)−1}, and b is the i^(th) modulation symbol.Additionally, an optional sequence denoted as h_(i,j) can be applied toeach pulse in the transmitted signal to ‘shape’ the spectrum of the:transmitted signal and to reduce spectral lines. The sequence, h_(i,j),is called a polarity scrambling sequence with values of either +1 or −1.Different amplitudes are also possible to further shape the spectrum.

FIG. 2 shows a conventional coherent TH-IR receiver 200. The receiverincludes an automatic gain control (AGC) unit 210 coupled to anamplifier 220 that is connected to the receive antenna 230. The receiveralso includes synchronization 240, timing control 250, channelestimation 260, MMSE equalizer 270, and decoder 280 units. Rake receiverfingers 290 input to an adder 295. Each rake receiver finger includes apulse sequence generator, correlator and weight combiner. The rakereceiver fingers reduce multipath interference.

An appropriate duration of the frame needs to be selected. A short frameduration decreases multiple access interference (MAI), and can alsoincrease performance in the presence of timing jitter, as described byS. Gezici, H. Kobayashi, H. V. Poor, and A. F. Molisch, “Effect oftiming jitter on the tradeoff between processing gains,” Proc. ICC 2004,pp. 3596-3600, 2004. On the other hand, interframe interference (IFI)can occur when the frame duration is shorter than a maximum excess delayof the channel impulse response. In conventional TH-IR systems, theframe duration is fixed and cannot be changed.

Therefore, it is desired to select adaptively the frame duration.

SUMMARY OF THE INVENTION

The invention provides a method and apparatus for adaptively determininga duration of a frame in a time-hopped, impulse radio (TH-IR) systemaccording to a current channel state and interference. In a multi-usersystem, interference is due to signals from other transceivers andnoise. Therefore, interference is a measure of the signal to noise andinterference ratio (SINR).

A receiver acquires channel state information (CSI), specifically, asmall-scale average power delay profile, as well as the average SNIR.The CSI and power delay profile are used to determine an optimal frameduration. The.;frame duration can be determined in either the receiveror the transmitter. The frame duration can be updated periodically asthe CSI and SNIR change over time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a timing diagram of prior art modulation techniques;

FIG. 1B is a timing diagram of prior art TH-IR modulation;

FIG. 2 is a block diagram of a prior art TH-IR receiver; and

FIG. 3 is flow diagram of a method for determining a frame durationaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Our invention provides a system and method for adaptive selection of aframe duration for transceivers in radio system. The frame durationdepends on the signal to noise and interference ratio (SNIR) and channelstate information (CSI). The method can be applied to coherenttransceivers in a time-hopped, impulse radio (TH-IR) system, as well asin transmitted-reference systems involving time hopping, and totime-hopping systems with incoherent transceivers. It should be notedthat at any one time the transceiver can either be transmitting orreceiving.

In a first step, the receiver estimates periodically 310 the CSI 311 ofthe, channel 301. This can be done in two ways. The receiver canestimate an instantaneous CSI or an average CSI. In the latter case, thereceiver estimates a small-scale, averaged power delay profile or anapproximation thereof. An accurate CSI is not necessary for theinvention. An approximation of the small-scale averaged power delayprofile or even just an estimate of the root-mean-square (RMS) delayspread can provide benefits. Whether to use the instantaneous or theaveraged CSI depends mostly on a ratio between symbol duration andcoherence time of the channel. In quasi-static channels, theinstantaneous CSI is preferred.

In a second step, the receiver estimates periodically 320 the SNIR 321of the channel 301. The SNIR can be estimated during a ‘quiet’ periodwhen no data is transmitted to the receiver. During this time, thereceiver is active and ‘listening’ to the channel. There are a greatnumber of ways to estimate CSI and SINR. The invention can work with anyconventional method to make these estimates. An overview of channel andinterference estimation can be found in J. G. Proakis, DigitalCommunications, fourth edition, McGraw-Hill, New York, 2001.

In an optional third step, the transceiver acquires periodically 330frame durations 331 used by other UWB transceivers 329. This can be doneby explicit transmissions by the other transceivers. For example, in thecontext of a network according to the IEEE 802.15.4 standard, a centralcoordinator device transmits beacons. The beacons contain the framedurations for all other devices-under the control of the coordinatordevice.

After the CSI and the SNIR have been estimated, an optimum frameduration 341 is determined 340. The frame duration can be determined ineither the transmitter or the receiver. The optimum frame durationminimizes the RMS error between a training signal and the receivedsignal, coded or uncoded bit error rate (BER), or other suitablecriteria. For example, the BER for a transmitted-reference scheme in thepresence of noise only is described by S. Gezici, F. Tufvesson, and A.F. Molisch, “On the performance of transmitted-reference impulse radio”,Proc. Globecom 2004. Alternatively, the optimum frame duration isdetermined from the: BER or RMS error from transmitted data. By‘dithering’ the frame duration in the : transmitter, the transceiver candetermine whether a smaller or larger frame duration improves the BER.This information is then supplied to the transmitter, and the frameduration is adapted accordingly.

We also optimize the time hopping (TH) sequence for the optimum frameduration. Conventionally, the TH sequence is preselected and optimizedfor a predetermined fixed frame duration. The preselected TH sequenceattempts to minimize the number of collisions of pulses per symbol,irrespective of varying relative delays between different transceivers.

The invention adaptively selects 350 a TH sequences 351 that retainsgood ‘collision’ properties when truncated to shorter durations. Adiscrete set of sequences 349 with different lengths can be used. Thetransmitter selects from this set of sequences the optimum sequence 351for the optimum frame duration 341.

EFFECT OF THE INVENTION

The selection of the frame duration according to the invention reducesinterframe interference and multiple access interference. Depending onthe environment in which the system is operating, the invention adjuststhe frame duration to minimize interframe interference, while at thesame time retaining good multiple access capabilities.

Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications may be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

1. A method for determining a duration of a frame in a time-hopping,impulse radio system, comprising: estimating periodically channel stateinformation of a channel between two transceivers; and determining aframe duration according to the channel state information.
 2. The methodof claim 1, further comprising: estimating periodically a signal tonoise and interference ratio; and determining the frame durationaccording to the a signal to noise and interference ratio.
 3. The methodof claim 1 in which the channel state information is instantaneous. 4.The method of claim 3, in which the channel state information is basedon an instantaneous power delay profile.
 5. The method of claim 1, inwhich the channel state information is averaged over time.
 6. The methodof claim 5, in which the channel state information is based on asmall-scale, averaged power delay profile.
 7. The method of claim 1, inwhich the channel state information is based on a root-mean-square (RMS)delay spread.
 8. The method of claim 2, in which the signal to noise andinterference is estimated during quiet periods when no data istransmitted.
 9. The method of claim 1, further comprising: acquiringperiodically other frame durations used by other transceivers of theradio system; and determining the frame duration according to the otherframe durations.
 10. The method if claim 9, in which the other framedurations are explicitly transmitted.
 11. The method of claim 1, inwhich the frame duration is determined in a receiver, and furthercomprising: transmitting the frame duration to a transmitter.
 12. Themethod of claim 2, in which the channel state information and the signalto noise and interference ratio are estimated in a receiver, and furthercomprising: transmitting the channel state information and the signal tonoise and interference ratio to a transmitter; and determining the frameduration in the transmitter.
 13. The method of claim 1, in which theframe duration minimizes a RMS error between a training signal and areceived signal.
 14. The method of claim 1, in which the frame durationminimizes a coded bit error rate.
 15. The method of claim 1, in-whichthe frame duration minimizes an uncoded bit error rate.
 16. The methodof claim 1, in which the frame duration minimizes a RMS error oftransmitted data.
 17. The method of claim 1, in-which the frame durationminimizes a bit error rate of transmitted data.
 18. The method of claim15, further comprising: dithering the frame duration in a transmitter.19. The method of claim 1, further comprising: selecting periodically atime hopping sequence according to the frame duration.
 20. Atime-hopped, impulse radio system, comprising: a receiver configured toestimate periodically channel state information and a signal to noiseand interference ratio of a channel, and the receiver furthercomprising: means for determining a frame duration according to thechannel state information and signal to noise and interference ratio;and means for transmitting the frame duration; and a transmitterconfigured to receive the frame duration.
 21. A time-hopped, impulseradio system, comprising: a receiver configured to estimate periodicallychannel state information and a signal to noise and interference ratioof a channel, and the receiver further comprising: means fortransmitting the channel state information and the signal to noise andinterference ratio; and a transmitter configured to determine a frameduration according to the channel state information and the signal tonoise and interference ratio.